Ghost Knife needs a BIG tank. They are really beautiful fish, don't work with a lot of tankmates, narrow choices.
Also won't really probably work with a Betta but cool fish I think of would be Kuhli loaches and also one of the really neat fish but you need to select tank mates and lighting and decor thoughtfully for their best interests, carefully are Glass Catfish. watch some videos on them, Smaller for a smaller space but really cool. They have an iridescent rainbow effect across their backs. They move in the water in a group and sway in place similar to a Ghost Knife. Though they would prefer a larger tank than a 20, say 29 Long or 40G breeder.
Great move researching ahead before making stocking selections.